Bending Test

Bending tests are carried out to obtain information on the bending properties of materials intended for industrial use or for research and development. In doing so, different checking fixtures are used.

A bending test (bending tensile test) is a method of testing materials for their bending strength and other important properties. Destructive materials testing is used for plastics, fiber-reinforced plastics (FRP), metals and ceramic materials. Bending tests are similar in their sequence. Depending on the number of pressure points and the support of the test specimen, a distinction is made between the following:

• 1-point bending test
• 3-point bending test
• 4-point bending test

In bending tests, standardized, mostly cylindrical specimens are placed in the center of the checking fixture. The rounded support rollers (bearings) are arranged parallel to each other at a certain distance (support width). The diameter of the cylindrical specimen is proportional to the support width of the bearings. The test punch, which moves down slowly and at a constant speed, loads the sample with increasing force until it breaks or reaches the previously determined deformation. The maximum load exerted during the bending test is called breaking force.

During the test, the values of the bending force and deflection are recorded. The material characteristics are then determined. The entire test sequence is shown in a stress-strain curve and can also be recorded with a video camera. Bending tests are performed to obtain information about the bending behavior of the tested material from the single-axis bending stress. In case of brittle materials, the bending strength is determined this way. Dealing with ductile materials, the limit yield point, the greatest possible bending angle as well as Young’s modulus are determined, in case of an elastic deformation.

During materials testing using a bending test, modern optical measuring systems with high-resolution cameras provide precise images of the test specimen. For the documentation of flat specimen, devices with a single camera are usually sufficient. More complex sample geometries can be accurately measured using two cameras. The material tester first applies a stochastic dot pattern to the sample or uses the existing surface structure. Optical measuring systems use image correlation algorithms: In the high-resolution images, they recognize the deformation caused by the bending test and then calculate the deflection using the pixel coordinates of the dot pattern.

If the bending stress in the sample made of a ductile material is lower than the limit stress of the plastic deformation, the bending stress is exclusively elastic. As the bending stress increases, the yield strength (critical stress) is exceeded first in the peripheral areas of the specimen. These areas then deform plastically (the so-called material flow). The limit yield point is the limit bending stress up to which easily deformable materials can be loaded by bending without permanent deformation in the marginal area.

The moment this kind of deformation occurs can be determined directly from the test punch: The deflection is measured in relation to the force applied. The determined values are shown in a deflection-force diagram. With deflection increasing steadily, more and more internal areas of the specimen are involved in the plastic deformation. This is a result of the stress increase. For steels, for example, the limit yield point is between 10 to 20 percent higher than the yield strength due to the linear stress increase. If the yield strength is exceeded in the edge fibers during the bending test, the inner and exclusively elastically stressed fibers impede the flow movement.

Bending tests with ductile materials are different from those performed with brittle materials: Tough materials can be subjected to extreme plastic deformation, but they cannot be broken, no matter how strong the applied force is. In the worst case, the specimen would be pulled through between the bearings. Therefore, a bending test with a ductile sample is finished when the yield point is exceeded. The bending strength of ductile materials is determined by the point in time at which the plastic deformation occurs.

Specimens made of brittle materials show a different bending behavior during materials testing. They break without clearly visible material flow behavior. Therefore, the determination of the limit yield point is more complicated for brittle materials. In order to be able to determine the bending strength nevertheless, the maximum bending stress at which the sample breaks is determined. However, the bending strength is a fictitious value that is not identical to the bending stress actually occurring in the material. Another characteristic of bending tests with brittle materials is the fracture deflection. This technical term describes the greatest possible deflection of a specimen shortly before fracture.

The fracture deflection depends on the support width: Larger distances between the bearings allow for greater deflections. In order to check the strength of brittle materials, the bending test is often more suitable than the tensile test, because the materials are subjected to bending stress only. If this sample were to be subjected to a tensile test, it would break prematurely and measurement problems would occur. For certain brittle materials, the tensile test is therefore replaced by the bending test. According to DIN EN ISO 178, these critical materials include thermosetting sheets and molding materials, thermoplastic injection molding compounds and fiber-reinforced plastics.

When testing materials using a bending test, a distinction is made between the 1-, 3- and 4-point bending test, depending on the number of pressure points and the type of specimen support.

Bending tests are performed with standardized samples and three or four pressure points (3-point, 4-point bending test). They either result in the destruction of the specimen or in its plastic deformation (only with ductile materials). The latest generation of optical metrology delivers far more accurate results than conventional measuring procedures.